Classify contextual fragments with content kinds (#40180)

## What changed

- Require each `ContextualUserFragment` to provide a stable `<feature>.<name>`
  `ContentItemKind`.
- Add `AnnotatedContent` and `RenderedFragment` so rendered text, its role, and
  its classification can travel together to API boundaries.
- Derive extension-owned world-state classifications from the extension ID and
  keep the skills catalog classification with its fragment implementation.

## Testing

- Verify that an extension-owned world-state section renders with an
  `<extension-id>.instructions` content kind.

GitOrigin-RevId: e46b74a0bb41e0b6112667c9d36bc9e7f2714451

codex-rs/ext/skills/src/extension.rs

@@ -9,7 +9,6 @@ use codex_exec_server::FileSystemSandboxContext;
 use codex_exec_server::LOCAL_ENVIRONMENT_ID;
 use codex_exec_server::ResolvedSelectedCapabilityRoot;
 use codex_extension_api::ConfigContributor;
-use codex_extension_api::ContentItemKind;
 use codex_extension_api::ContextContributor;
 use codex_extension_api::ContextualUserFragment;
 use codex_extension_api::ExtensionData;
@@ -238,10 +237,7 @@ where
             rendered
                 .fragment
                 .map(|fragment| {
-                    PromptFragment::developer_capability(
-                        fragment.render(),
-                        ContentItemKind("skills.catalog".to_string()),
-                    )
+                    PromptFragment::developer_capability(fragment.render(), fragment.content_kind())
                 })
                 .into_iter()
                 .collect()

codex-rs/ext/skills/src/fragments.rs

@@ -1,4 +1,5 @@
 use codex_extension_api::ContextualUserFragment;
+use codex_protocol::models::ContentItemKind;
 use codex_protocol::protocol::SKILLS_INSTRUCTIONS_CLOSE_TAG;
 use codex_protocol::protocol::SKILLS_INSTRUCTIONS_OPEN_TAG;
 
@@ -40,6 +41,10 @@ impl ContextualUserFragment for AvailableSkillsInstructions {
         "developer"
     }
 
+    fn content_kind(&self) -> ContentItemKind {
+        ContentItemKind("skills.catalog".to_string())
+    }
+
     fn markers(&self) -> (&'static str, &'static str) {
         Self::type_markers()
     }
@@ -73,6 +78,10 @@ impl ContextualUserFragment for SkillInstructions {
         "user"
     }
 
+    fn content_kind(&self) -> ContentItemKind {
+        ContentItemKind("skills.selected_skill_instructions".to_string())
+    }
+
     fn markers(&self) -> (&'static str, &'static str) {
         Self::type_markers()
     }